High bit-rate (LH and/ULH) dense wavelength-division-multiplexing (DWDM) transmission systems require reliable, compact, and economical transmitters. FIG. 1 depicts a typical transmitter 100 for ON-OFF-keying, Return-to-Zero (RZ) transmission and consists of a semiconductor distributed-feedback (DFB) laser 102 followed by a pulse carver modulator (PCM) 104 and an electroabsorption data modulator (EAM) 106. The order of the EAM and PCM is reversible. The PCM is driven by an electronic clock 108 running at the line rate of the system (10 GHz, for example) and produces a train of RZ pulses from the DFB laser output to act as a carrier for data. A phase shifter 112 is also typically placed between the clock 108 and the PCM 104 to initialize transmitter timing. An electronic data pulse stream to be transmitted (consisting of, for example, a series of square electric pulses representing 1's and 0's of binary data D from data module 110) modulates the optical transmission of the EAM, and the data is encoded into an optical pulse train. The final output of the two modulators is an optically modulated data pulse train.
Two problems associated with such transmitters are maintaining the stringent requirements of the output wavelength and power stability and maintaining the correct timing between the two modulators for the pulse carving and data modulation. Temperature fluctuation in the field and the aging of the electronic devices cause the RF group delays of the drive circuits of the modulators to drift, resulting in timing misalignment. This timing misalignment increases the penalties in the data transmission and needs to be addressed for optimal performance of the optical communication systems. For example, one aspect of the penalties that may arise in system 100 is seen by inspection of the graphs shown in FIGS. 2A–2D. FIG. 2A depicts a case in which the optical pulse from the PCM enters the EAM too early and leads the data pulse. In such a circumstance, a positive chirp is introduced into the data (a change in frequency Δω and as seen by the dotted line above the data and timing curves in FIG. 2A). The spectral analysis of the timing conditions of FIG. 2A is shown in FIG. 2B wherein a center frequency (CF) is flanked by asymmetrical lower sideband (LSB) and upper sideband (USB). The reverse conditions (wherein the optical pulse lags behind the data pulse is seen in FIG. 2C. In this condition, spectral analysis (as shown in FIG. 2D) reveals that the asymmetry of the upper and lower sidebands still exists, but is reversed from the previous condition. In an optimal and desired condition, the data pulse and the pulsed optical output of the PCM share a common center point with respect to time (denoted by (x) in FIGS. 2A and C). In such a condition, chirp is minimized and the spectral analysis reveals symmetrical upper and lower sideband modulation levels. The timing drift in real devices tends to be random and thus the optical data will suffer from random chirping without an active management of the timing between the PCM and EAM.
Therefore, it is desired to have an apparatus for LH and ULH DWDM transmission that is capable of synchronizing the pulse carver modulator and electroabsorption data modulator and a concomitant method for establishing such operational conditions.